A key telephone system is a small local telephone system in a small office complex or home having a plurality of telephones which are interconnected, wherein each of the telephones has access to a smaller plurality of lines connected to a local telephone central office. Incoming telephone calls are routed selectively to the desired telephone by causing audible sounding of a ringer or the like in the selected phone and, at the same time, causing a lamp associated with this phone (such as a lamp disposed in a button in the selected phone) to flash.
The power to actuate the lamps and ringers is provided from a local source, typically an alternating current source. Should the supply of power to the system be cut off, these lamps and ringers become inoperative, and users of the system have no way of knowing that incoming calls are to be answered even though the system otherwise functions normally in originating or receiving calls. Consequently, incoming calls are not answered under such conditions.
The present invention is directed toward apparatus for use with key telephone systems which enables users of the system to receive and answer incoming calls when the supply of power from a local source is interrupted in the same manner as if the supply had not been interrupted. The apparatus includes substitute lamps and an audible alarm which are enabled when the supply is interrupted to function in such conditions in the same manner as the lamps and ringers utilized in normal operating conditions.